Optically activated reactions initiate biological processes such as photosynthesis or vision, but can also control polymerization, catalysis or energy conversion. Methods relying on the manipulation of light at macroscopic and mesoscopic scales are used to control on-surface photochemistry, but do not offer atomic-scale control. Here we take advantage of the confinement of the electromagnetic field at the apex of a scanning tunnelling microscope tip to drive the phototautomerization of a free-base phthalocyanine with submolecular precision. We can control the reaction rate and the relative tautomer population through a change in the laser excitation wavelength or through the tip position. Atomically resolved tip-enhanced photoluminescence spectroscopy and hyperspectral mapping unravel an excited-state mediated process, which is quantitatively supported by a comprehensive theoretical model combining ab initio calculations with a parametric open-quantum-system approach. Our experimental strategy may allow insights in other photochemical reactions and proof useful to control complex on-surface reactions.

光激活反应启动光合作用或视觉等生物过程，但也可以控制聚合、催化或能量转换。依赖于宏观和介观尺度上的光操纵的方法用于控制表面光化学，但不提供原子尺度的控制。在这里，我们利用扫描隧道显微镜尖端顶端电磁场的限制，以亚分子精度驱动游离碱酞菁的光互变异构化。我们可以通过改变激光激发波长或通过尖端位置来控制反应速率和相对互变异构体数量。原子分辨尖端增强光致发光光谱和高光谱映射揭示了激发态介导的过程，该过程得到了从头计算与参数开放量子系统方法相结合的综合理论模型的定量支持。我们的实验策略可能有助于了解其他光化学反应，并证明有助于控制复杂的表面反应。